This invention pertains generally to the field of micro-electro-mechanical systems and to accelerometers and shock sensors.
Accelerometers are one of the most significant applications of micro-mechanics and have been the subject of active research for more than two decades. See, e.g., L. M. Roylance, et al., xe2x80x9cA Batch-Fabricated Silicon Accelerometerxe2x80x9d IEEE Trans. Elec. Dev., ED-26, 1979, pp. 1911-1917. Shock sensors are accelerometers which are designed to respond to specific threshold levels of acceleration. A typical design for a shock sensor includes a proof mass supported by a flexible suspension. Generally, at a pre-selected level of acceleration, the resulting deflection of the proof mass will cause it to contact an electrical element to close a switch, much like a relay. Since each sense element triggers at a single threshold, it is necessary to use arrays of sense elements to cover a wide dynamic range. The discrete outputs generated permit these devices to operate with a relatively simple interface circuit, which can be designed to have minimal power dissipation. An example is described in A. Selvakumar, et al. xe2x80x9cLow Power, Wide Range Threshold Acceleration Sensing System,xe2x80x9d IEEE MEMS ""96 pp. 186-191. Low power dissipation permits long term operation from a small battery. One potential application for such devices is for use with environmental monitoring systems of the type which monitor temperature, pressure, humidity and a number of other variables, and which are kept normally in a sleep mode to preserve battery life. The shock sensors can be utilized to wake up the environmental monitoring system when a shock is detected. Other potential applications for shock sensors include air bag deployment systems, munitions arming, monitoring of seismic activity, and monitoring of shocks imposed on fragile shipments.
In most shock sensor designs, the proof mass returns to its rest position once the acceleration is removed. Devices in which the deflection is latched have also been reported. See R. Ciarlo, xe2x80x9cA Latching Accelerometer Fabricated by the Anisotropic Etching of (110) Oriented Silicon Wafers,xe2x80x9d J. Micromech. Microeng., Vol. 2, 1992, pp. 10-13; X.-Q. Sun, et al., xe2x80x9cA Surface Micromachined Latching Accelerometer,xe2x80x9d Transducers ""97, pp. 1189-92. Such latching devices may be useful for certain applications, but with some limitations on sensitivity and reusability.
Prior micromachined shock sensors have typically detected out-of-plane accelerations. One substrate-plane sensing device has been reported, but targeting very high g-forces. P. F. Man, et al., xe2x80x9cSurface Micromachined Shock Sensor for Impact Detection,xe2x80x9d Solid-State Sensor and Actuator Workshop, Hilton Head, N.C., 1994, pp. 156-159. In general, substrate-plane sensing is desirable since it will simplify mounting and alignment and can also simplify bidirectional sensing (i.e., along the positive and negative direction of the sense axis) because electrodes can be in the same plane as the proof mass and only one structural and electrical area is required.
A persistent challenge for micromachined shock sensors has been the closing and opening of the electrical contacts. See, e.g., Y. Loke, et al., xe2x80x9cFabrication and Characterization of Silicon Micromachined Threshold Accelerometers,xe2x80x9d Sensors and Actuators A, Vol. 29, 1991, pp. 235-240; A. Selvakumar, supra. If the proof mass is very small, its momentum may fail to break through surface films that may inadvertently form on the electrical contact. In addition, if the suspension for the proof mass is too weak, forces established during contact may prevent the retraction of the proof mass.
In accordance with the invention, a micromachined shock sensor is provided which can be formed with dimensions of a few millimeters on a side or less in an efficient and cost-effective manner. The shock sensor can be utilized to provide discrete output signals indicating acceleration levels over a wide range of accelerations with accuracy and repeatability.
A shock sensor in accordance with the invention capable of detecting multiple levels of acceleration includes a substrate having a surface on which are formed an array of acceleration sensing units. Each sensing unit comprises a mount fixed to the substrate, a cantilever beam extending from the mount over the substrate surface and free to bend in a plane above the substrate surface, and a proof mass fixed to the cantilever beam and supported by the cantilever beam above the surface of the substrate to permit translation of the proof mass and bending of the cantilever beam in a plane parallel to the substrate surface. First and second sensing electrodes are formed on the substrate on opposite sides of the proof mass and adjacent to the proof mass and have contact elements that are spaced by a sensing gap from the proof mass. Displacements of the proof mass in response to accelerations brings the proof mass into contact with one or the other of the electrodes at a sufficient acceleration level. Several sensor units have cantilever beam dimensions and proof mass dimensions that are selected to provide contact between the proof mass and the adjacent sensing electrodes at different levels of acceleration. An electrical conductor is formed on the substrate electrically connected to all of the first sensing electrodes and an electrical conductor is formed on a substrate that is electrically to all of the second sensing electrodes. The various levels of acceleration may be detected by applying appropriate voltages (e.g., positive and negative DC voltages) to the first and second sensing electrodes and then detecting any voltage that is applied to the proof mass as it makes contact with a sensing electrode, preferably by making an electrical connection to the mount connected by a conductive cantilever beam to the conductive proof mass.
The acceleration sensing units may further include a test electrode formed on the substrate adjacent to the proof mass on one side thereof and adjacent to the first sensing electrode. Application of a voltage between the test electrode and the proof mass electrostatically draws the proof mass toward the test electrode until, at a sufficient voltage, the proof mass contacts the first sensing electrode. The test electrode may be electrically connected to the second sensing electrodes so that a common electrical connection may be provided to both the second sensing electrodes and to the test electrodes.
The mount, cantilever beam, proof mass, sensing electrodes and test electrodes may be formed on the substrate surface of electroplated metal by micromechanical manufacturing processes. The mount, cantilever beam and proof mass are preferably integrally formed together of electrically conductive material, and the proof mass and cantilever beam preferably have the same height. The microstructural elements preferably have a height of 500 xcexcm or less and occupy an area on the substrate of less than about one square centimeter.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.